The most common reasons are that one of the sensors has been accidentally knocked and the beam is out of alignment, or something is blocking the path of the beam between the sensors. A spiderweb with debris in it is sometimes enough to block the beam. Check for these two things first.
It can also be caused by dirt on the sensor eyes, loose or corroded wire connections at the sensors or the motor head, or just that the sensors have gone bad and need to be replaced. We recommend unplugging the garage door opener and then re-plugging it back in after any repairs or adjustments to reset the system.